In 1937, Isidor Isaac Rabi (pronounced RAH-bee) developed a method to measure the magnetic moments (spin and magnetic characteristics) of atomic nuclei. His technique was fundamental to subsequent atomic beam experiments, leading to development of magnetic resonance imaging (MRI) in medicine, guidance systems for missiles and satellites, and Rabi's Nobel Prize for Physics in 1944. In 1945 he proposed construction of the first atomic clock, and a founding member of the Brookhaven National Laboratory in 1947, and Conseil Européen pour la Recherche Nucléaire (CERN) in 1954.

He was born in a Polish town that was then part of the Austro-Hungarian empire, and came to America with his family when he was an infant, where his father found work as a tailor. "Had we stayed in Europe", he later said, "I probably would have become a tailor". Instead he earned a degree in chemistry and worked for several years in an industrial laboratory, before returning to college to study physics.

He was involved in wartime improvements to radar technology, and though he declined Robert Oppenheimer's invitation to work on the Manhattan Project, he visited the Los Alamos laboratory numerous times and provided significant informal advice and valuable troubleshooting expertise. He later explained that he had moral qualms about using the atom as an ultimate weapon, and said he believed that World War II could be won without the bomb. After the war he became an outspoken opponent of atomic weaponry, which he described as "necessarily an evil thing" and "wrong on fundamental ethical principles".